Data from the literature and previous in vitro research conducted in the investigators' laboratory (INSERM U413/EA4310, University of Rouen) suggest that adrenal corticosteroid secretion might be controlled by a paracrine mechanism involving serotonin type IV receptor (5-HT 4). L-lysine, a common amino-acid has been shown to act as a 5-HT4 agonist in vitro as well as in vivo. In the present physiology trial, plasma aldosterone and cortisol levels will be measured under treatment with aprepitant versus placebo, in both basal conditions and after activation of the adrenocortical function by various stimuli, including upright posture, metoclopramide, and after a 3 days salt-free diet. All healthy volunteers will be given two substances (L-lysine and placebo) in a random order during two 13 days periods separated by a 14 day-wash-out. This study should allow to determine the role of 5-HT4 receptors in the control of corticosteroid production in normal man.